Semiconductor laser device

ABSTRACT

A semiconductor laser device in which semiconductor layers of an n-type cladding layer, a quantum well active layer 106, a p-type cladding layer, and an intermediate layer are formed on an n-type GaAs substrate in successive order, and a mixed-crystal is formed in a region except the semiconductor layers of the contact layer and the lower part of the contact layer by diffusing Si into the structure from above the intermediate layer, characterized in that the contact layer and the intermediate layer are made of n-type or nonconductive semiconductor material, and a p-type low-resistance region, formed by diffusing Zn into the structure from above the contact layer, is profiled so as not to overlap with the mixed-crystal region formed by Si diffusion.

BACKGROUND OF THE INVENTION

The present invention relates to a visible buried semiconductor laserdevice manufactured by the mixed-crystal forming technique based onimpurity diffusion.

When an impurity such as Zn is thermally diffused or ion planted intothe superlattice, which consists of extremely thin semiconductor layers,several to several tens nm thick, of different compositions such asAlGaAs--GaAs, the superlattice is destroyed and transformed into auniform mixed-crystal. This fact is known. When the mixed-crystal isformed, refractivity, forbidden band, and the like are varied. Thisnature can be used for confining light and carriers therein. The buriedsemiconductor laser device, manufactured by the mixed-crystal formingtechnique based on impurity diffusion, has various advantages: lowthreshold current value, high efficiency, stable transverse mode, andeasy integration. For this reason, AlGaAs series as a material for thenear-infrared semiconductor laser is frequently used for the buriedsemiconductor laser device.

Appl. Phys. Lett. 54, p2136 (1989) describes the mixed-crystal formingtechnique based on the impurity diffusion in A1GaInP series as amaterial for the visible semiconductor laser device, in which theimpurity is Zn. When Zn is diffused, a natural superlattice of A1GaInPas a material for the visible semiconductor laser device is transformedinto a mixed-crystal, but mutual diffusion of Al and Ga is notremarkable. The natural superlattice follows. When A1GaInP is crystalgrown by a MOCVD method, the group III atoms are regularly arrayed intothe natural superlattice. This phenomenon is described in Appl. Phys.No. 9 of Volume 58, p1360 (1989). Where the natural superlattice istransformed into a mixed-crystal, the forbidden band width differencefor confining carriers and the refractivity difference for confininglight cannot be rendered satisfactorily large between a mixed-crystalregion and a non-mixed-crystal region.

To efficiently confine carriers, it is necessary to greatly vary theforbidden band width and the refractivity between a mixed-crystal regionand a non-mixed-crystal region, by causing Al and Ga to mutuallydiffuse.

Journal of Applied Physics, vol. 66, p482 (1989) describes asemiconductor laser device manufactured by the mixed-crystal formingtechnique based on the Si diffusion in AlGaInP series. A Si-diffusionbasis, buried visible semiconductor laser device is simpler manufacturethan the ridge-stripe type visible semiconductor laser device.

The laser described in the paper has poor laser characteristics sinceduring the manufacturing process, particularly during the Si diffusingprocess, many dislocations and defects are caused. Our observation by atransmission electron microscopy showed that these dislocations anddefects are concentrated mainly in the surface region of the secondconductivity type GaInP intermediate layer in the Si diffused region.

The influence of the dislocations and defects on the semiconductor laserdevice will be described. FIG. 9 shows an AlGaInP buried laser, which ismanufactured by the mixed-crystal forming technique based on the Siimpurity diffusion disclosed in Published Unexamined Japanese PatentApplication No. Hei. 6-53604. In the figure, reference numeral 401designates an n-type side electrode; 402, an n-type GaAs substrate; 403,an n-type GaInP buffer layer; 404, an n-type AlInP cladding layer; 405,a GaInP active layer; 406, a p-type AlInP cladding layer; 407, a p-typeGaInP intermediate layer; 408, an Si diffusion source film; 409, an SiO₂diffusion protective film/current block layer; 410, a p-type sideelectrode; 411, a p⁺ type GaAs contact layer; 412, an Si diffusionregion; and 413, a surface region where many dislocations and defectsthat are caused by the Si diffusion are present.

In this type of the AlGaInP buried laser by the Si diffusion, ideally,the current injected from the p-type side electrode 410 is squeezed bythe pn junction formed along the boundary between the p-type GaInPintermediate layer 407 and the Si diffusion region 412. As a result, thecurrent is efficiently injected into only the active layer.

Actually, the current leaks through the boundaries 414 and 415 eachbetween the surface region 413 where many dislocations and defects arepresent and the p-type GaInP intermediate layer 407. In this situation,the current is inefficiently injected into the active layer. The resultis high threshold current value, low efficiency,. and poor temperaturecharacteristic.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to improve an AlGaInPburied laser, manufactured by the mixed-crystal forming technique basedon the Si impurity diffusion, so as to cause no leak current, to reducethe threshold current value, and to increase the efficiency.

To achieve the above object, there is provided a semiconductor laserdevice, comprising:

a) a substrate;

b) a first cladding layer made of semiconductor material of a firstconductivity type and layered on the substrate;

c) a quantum well active layer made of semiconductor material andlayered on the first cladding layer;

d) a second cladding layer layered on the quantum well active layer, thesecond cladding layer being made of semiconductor material of a secondconductivity type that is opposite to the first conductivity type;

e) an intermediate layer made of semiconductor material of the firstconductivity type or nonconductive type semiconductor, the intermediatelayer being layered on the second cladding layer;

f) a contact layer made of semiconductor material of the firstconductivity type or nonconductive type semiconductor, the contact layerbeing layered on a part of the intermediate layer;

g) a mixed-crystal region of the first conductivity type being formed ata location deviated sideways from a region under the contact layer,while ranging from the intermediate layer to the quantum well activelayer; and

h) a low resistance region of the second conductivity type extended overa range from the contact layer, through the intermediate layer, to partof the second cladding layer, the low resistance region being isolatedfrom the mixed-crystal region by the intermediate layer.

The layers constituting the semiconductor laser device will be describedbelow.

Intermediate layer:

The intermediate layer is a mixed-crystal layer or a superlattice layerbetween a cladding layer and a contact layer, and also called a bufferlayer. In the vertical structure of the semiconductor laser device, theintermediate layer functions as follows. When the intermediate layer isinserted between the cladding layer and the contact layer, the banddiscontinuity quantity between the cladding layer and the contact layeris reduced, a depletion layer present in the interface is reduced inthickness, and as a consequence a series resistance of the semiconductorlaser device is reduced. In the present invention, the conductivity type(n or p type) of the intermediate layer is nonconductive or opposite tothe conductivity type of the cladding layer that is in contact with theunderside of the intermediate layer. The intermediate layer is a GaInPor AlGaInP mixed-crystal layer or an AlInP/GaInP superlattice layer.

Contact layer:

The contact layer is a layer on which an electrode is to be formed. Asemiconductor of small band gap is used for the contact layer, in orderto reduce a contact resistance created in the interface between theelectrode metal and the semiconductor. The contact layer is made ofGaAs.

Cladding layer:

The cladding layer confines carriers within the active layer, thereby tomake it easy to recombine carriers, and confines light that is caused inthe active layer by the recombination of carriers and stimulatedemission, in the vertical direction (perpendicular to the substrate). Astructure of the semiconductor laser device, which consists of acladding layer, an active layer, and a cladding layer, is usually calleda double heterostructure. This structure is a basic structure forcausing a laser oscillation. AlInP and AlGaInP are typical examples ofthe double heterostructure.

Quantum well active layer:

In this layer, light is emitted by the recombination of carriers orstimulated emission, and resonates in the layer. GaInP is an example ofthe quantum well active layer.

Formation of the low resistance region can be controlled by properlyselecting the diffusion conditions of the first impurity and the secondimpurity, the size of the opening, a positional relationship between theopening and the first impurity diffusion source, and the thickness ofthe contact layer and the intermediate layer. In the impurity diffusionprocess, the diffusion region depends on the diffusion length determinedby the diffusion condition such as the diffusion temperature and time,the position of the first impurity layer as a diffusion source and theopening through which the second impurity is supplied. The diffusionalmost isotopically progresses in the lateral direction and in thevertical direction. Each of the diffusion lengths of the first impurityand the second impurity can be controlled independently by adjusting thediffusion conditions, because the diffusion temperature of the firstimpurity is not equal to the diffusion temperature of the secondimpurity.

A desired low resistance region may be formed if the relationships amongthe size of the opening of the diffusion protective layer, and thethickness of the contact layer and the intermediate layer are selectedin the following way on the basis of this fact (FIG. 1B).

The diffusion conditions of the first and the second impurity, aspecific size of the opening or contact hole, and the positionalrelationship between the contact hole and the first impurity diffusionsource are selected so as to satisfy the following inequality

    l.sub.1 +l.sub.2 <1

where l₁ : length of the diffusion of the first impurity in thedirection parallel to the surface of the intermediate layer

l₂ : length of the diffusion of the second impurity in the directionparallel to the surface of the intermediate layer

l: distance between the side wall of the contact hole and the end of thefirst impurity diffusion source.

The diffusion length c of the second impurity in the direction verticalto the substrate surface is uniquely determined as described above. Thetotal thickness d of the contact layer and the intermediate layer, orthe distance from the uppermost surface of the semiconductor layer intowhich the second impurity is diffused to the second cladding layer, mustbe selected so as to satisfy the following inequality

    d<c.

In a buried laser such as AlGaInP laser manufactured by themixed-crystal forming technique based on the Si impurity diffusion,which incorporates the present invention, a current path avoids thesurface region of the Si diffusion region in the p-type GaInPintermediate layer where the dislocations and defects are concentricallypresent. Therefore, there is successfully eliminated the leak currentflowing through the dislocations and defects. Low threshold currentvalue, high efficiency, good temperature characteristic, and the likeare realized in the resultant visible semiconductor laser device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional view showing a buried semiconductor laserdevice according to a first embodiment of the present invention.

FIG. 1B is a cross sectional view showing positional relationships amongthe regions and layers in the structure of the semiconductor laserdevice of FIG. 1A.

FIGS. 2A and 2B are cross sectional views for explaining a leak currentcontrol mechanism in the intermediate layer in the first embodiment.

FIGS. 3A and 3B are cross sectional views showing procedural steps ofmanufacturing the buried semiconductor laser device according to thefirst embodiment of the present invention.

FIGS. 4A and 4B are cross sectional views showing procedural steps ofmanufacturing the buried semiconductor laser device, which follow theprocedural steps of FIGS. 3A and 3B.

FIGS. 5A and 5B are cross sectional views showing procedural steps ofmanufacturing the buried semiconductor laser device, which follow theprocedural steps of FIGS. 4A and 4B.

FIGS. 6A and 6B are cross sectional views showing procedural steps ofmanufacturing the buried semiconductor laser device, which follow theprocedural steps of FIGS. 5A and 5B.

FIGS. 7A and 7B are cross sectional views showing a procedural step ofmanufacturing the buried semiconductor laser device, which follow theprocedural steps of FIGS. 6A and 6B.

FIG. 8 is a sectional view in perspective of a vertical resonance typesemiconductor laser device according to a third embodiment of thepresent invention.

FIG. 9 is a cross sectional view useful in explaining a leak currentgeneration mechanism in a conventional buried semiconductor laserdevice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

FIG. 1A is a cross sectional view showing a semiconductor laser deviceaccording to an embodiment of the present invention. The semiconductorlaser device described in the present embodiment is a semiconductorlaser device of the edge-emitter type in which the resonator extends inthe direction (vertical to the paper surface) parallel to the plane of asemiconductor substrate.

In the figure, reference numeral 101 designates an n-type side electrode101; 102, an n-type GaAs substrate; 103, an n-type GaAs buffer layer;104, an n-type GaInP buffer layer; 105, an n-type (Al_(x) G_(1-x))₀.5In₀.5 P (0.5<X≦1) cladding layer; 106, a GaInP active layer; 107, ap-type (Al_(x) Ga_(1-x))₀.5 P (0.5<X≦1) cladding layer; 108, an Sidiffusion region; 109, an n-type or nonconductive (Al_(x)Ga_(1-x))₀.5)In₀.5 P (0≦X<0.5) intermediate layer; 110, an Si diffusionsource film; 111, an SiO₂ current block layer; 112, a p-type sideelectrode; 113, an n-type or nonconductive GaAs contact layer; and 114,a Zn diffusion region.

It is noted that in the semiconductor laser device, 1) the intermediatelayer 109 and the n-type or nonconductive GaAs contact layer 113 are ofthe n-type or nonconductive type, and 2) the Zn diffusion region 114 isprofiled so as not to overlap with the Si diffusion region 108 (FIG.1A). Current injected from the p-type side electrode 112 flows avoidinga surface region of the intermediate layer 109 where dislocations andlattice defects, caused by the Si impurity diffusion, are concentricallypresent. Therefore, there is eliminated the current leaking throughthose dislocations and defects.

A mechanism that current injected from the p-type side electrode 112flows avoiding the surface region of the intermediate layer 109 wheremay dislocations and defects are caused by the Si impurity diffusion,will be described in detail with reference to FIGS. 2A and 2B. FIGS. 2Aand 2B are cross sectional views emphatically showing the intermediatelayer 109 in the semiconductor laser device of FIG. 1A. Numerals 115 and116 designate the surface regions of the intermediate layer 109containing many dislocations and lattice defects caused by the Siimpurity diffusion. FIG. 2B shows an enlarged, sectional view showing aportion 117 in the structure of FIG. 2A. Numeral 202 indicates aboundary between the intermediate layer 109 and the Si diffusion region108 where a small number of dislocations and defects are caused by theSi impurity diffusion. Numeral 201 indicates a boundary between theintermediate layer 109 and the surface region 115 containing manydislocations and defects caused by the Si impurity diffusion. When theintermediate layer 109 is of the p type, and a pn junction is formed inthe boundary 201, current leaks through the dislocations and defects asin the conventional art.

In the present invention, 1) the intermediate layer 109 is of thenegative conductivity type, and 2) an electron depletion region by thepn junction is formed in the boundary 203 between the Zn diffusionregion 114 and the intermediate layer 109 where no defect is present. Bythis structure, no current leaks through the dislocations and thedefects. If an undoped high resistance layer is used for theintermediate layer 109, this portion 117 forms a pin junction. Further,a self-align process to be given later allows the intermediate layer 109as the i layer to have a large width W of approximately 1 μm. Therefore,the resistance of the pin junction is satisfactorily large (proportionalto the square of the width W). The high resistance blocks the movement(indicated by numeral 204) of carriers from the Zn diffusion region 114to the boundary 202.

Incidentally, W=l-(l₁ +l₂) (FIG. 1B).

A method of manufacturing the semiconductor laser device thus structuredwill be described with reference to FIGS. 3 to 7.

To start, the following semiconductor layers are formed on an n-typeGaAs substrate 102 in successive order by MOCVD (metal organic chemicalvapor deposition) or GSMBE (gas source molecular beam epitaxy): ann-type GaAs buffer layer 103 of 0.2 μm, made of Si doped n-type GaAs, abuffer layer 104 of 0.2 μm thick, made of Si doped n-type Ga₀.5 In₀.5 P,a cladding layer 105 of 1 μm thick, made of Si doped n-type (Al_(x)Ga_(1-x))₀.5 In₀.5 P (0.5 X)≦1), an undoped active layer 106, a claddinglayer 107 of 0.5 μm thick, made of Zn doped p-type (Al_(x) Ga_(1-x))₀.5In₀.5 P (0.5<X)≦1), an intermediate layer 109 of 0.1 μm thick, made ofn-type or nonconductive (Al_(x) Ga_(1-x))₀.5 In₀.5 P (0≦X<0.5), and acontact layer 114 of 0.2 μm thick, made of undoped GaAs. In thestructure of the active layer, two of three Ga₀.5 In₀.5 P well layerseach of 10 nm thick are separated by two (Al₀.5 Ga₀.5)₀.5 In₀.5 Pbarrier layers, and these separated ones are each sandwiched by (Al₀.5Ga₀.5)₀.5 In₀.5 P wave guiding layers.

A stripe-like photoresist 301 of 7 μm wide is formed on the resultantstructure by photolithographic technique (FIG. 3A). Using thisphotoresist 301 as a mask, the GaAs contact layer 113 is selectivelyetched into a stripe of 3 μm wide by an etching solution as a mixture ofNH₄ OH, H₂ O₂ and H₂ O at the ratio of NH₄ OH: H₂ O₂ : H₂ O=1:2:100, forthe (Al_(x) Ga_(1-x))₀.5 In₀.5 P intermediate layer 109. The etchingsolution used here little etches the (Al_(x) Ga_(1-x))₀.5 In₀.5 Pintermediate layer 109. It etches only the side of the GaAs contactlayer 113. In other words, the width of the GaAs contact layer 113 canbe controlled.

Si diffusion source films 110 and 302 as impurity diffusion sources aredeposited, for example, approximately 5 nm on the entire surface of thestructure in vacuum of 1.3×10⁻² Pa or less by an electron beamheating/depositing apparatus, while the photoresist 301 is left as it is(FIG. 4A). In the deposition process, Si is not deposited on the uppersurface of the GaAs contact layer 113 under the photoresist 301, and aportion 303 of the upper surface of the intermediate layer 109 which islocated under the photoresist 301 and exposed as the result of the sideetching of the GaAs contact layer 113, since those portions are maskedwith the photo photoresist 301.

Thereafter, by removing the photoresist 301, the Si film 302 above theGaAs contact layer 113 is removed by a lift-off method (FIG. 4B).

Then, an SiO₂ film 111 of 50 nm thick as a surface protective film isformed on the entire upper surface of the structure (FIG. 5A).

The structure, together with phosphorus, is put into a quartz tube, andsealed. Then, it is placed at 850° C. for 3 hours in an electricfurnace. Through the heat treatment, Si is diffused till it reaches then-type (Al_(x) Ga_(1-x))₀.5 In₀.5 P (0.5X≦1) cladding layer 105, wherebyan Si diffusion region 108 is formed (FIG. 5B). The Si diffusion region108 is of the n type, and forms a mixed-crystal in the intermediatelayer 109 and the p-type (Al_(x) Ga_(1-x))₀.5 In₀.5 P (0.5<X≦1) claddinglayer 107, and in the p-type (Al_(x) Ga_(1-x))₀.5 In₀.5 P (0.5<X≦1)cladding layer 107, the GaInP active layer 106, and the n-type (Al_(x)Ga_(1-x))₀.5 In₀.5 P (0.5<X≦1) cladding layer 105. As the result of thelateral Si diffusion, the Si diffusion region is extended more inwardthan the Si vapor deposition end by l₁. In the process of the presentembodiment, l₁ is approximately 1 μm.

Subsequently, the structure is taken out of the quartz tube, and theSiO₂ layer 111 is coated with photoresist 304. A window 305 is formed bynormal photolithographic technique (FIG. 6A). The width of the window305 is narrower than the width 306 of the GaAs contact layer 113. TheSiO₂ layer 111, located right under the window 305, is removed withbuffered fluoric acid, and the photoresist 304 is removed with organiccleaning fluid (FIG. 6B).

The structure, together with zinc, phosphorus, and arsenic, is put intoa quartz tube and sealed. Then, it is placed at 550° C. for 20 minutesin an electric furnace. Through the heat treatment, with the SiO₂ film111 as a mask, Zn is selectively diffused into the n-type or undopedGaAs contact layer 113, to thereby form an Zn diffusion region 114 (FIG.7A). The temperature and time of the heat treatment may be properlyselected if the diffusion front of the Zn diffusion region 114 reachesthe p-type Al₀.5 In₀.5 P cladding layer 107. Since the Zn diffusion isisotropic, the lateral diffusion distance is substantially equal to thevertical diffusion distance (approximately 0.1 μm). Since the width W2is large, 1 μm, there is less chance that the Zn diffusion region 114overlaps with the Si diffusion region 108.

It is also possible to diffuse Zn into the region as illustrated in FIG.7B, because the Zn diffusion region should not overlap the Si diffusionregion.

Thereafter, a p-type side electrode 112 and an n-type side electrode 101are respectively vapor deposited over the top and bottom sides of thestructure, to complete a semiconductor laser device as shown in FIG. 1A.

(Second Embodiment)

In a second embodiment of a semiconductor laser device according to thepresent invention, the n-type or nonconductive intermediate layer 109 inFIG. 1A is substituted by a superlattice layer consisting of n-type(first conductivity type) or nonconductive GaInP thin film layers andAlInP thin film layers that are alternately layered. Provision of thethus structured superlattice layer forms a mini-band in the intermediatelayer. By adjusting the thickness of the GaInP thin film layer and thethickness of the AlInP thin film layer, the effective band gap in theintermediate layer is set to be wider than the band gap of the activelayer, but to be narrower than the band gap of the cladding layer of thep type (second conductivity type). By so adjusting, the thresholdvoltage of the pn junction or the pin junction in the region shown inFIG. 2B is increased above the threshold voltage of the active layer.The leak current restricting effect is improved. The band discontinuityquantity of the contact layer and the p-type (second conductivity type)cladding layer is reduced, thereby reducing the series resistance.

(Third Embodiment)

In the first and second embodiments, the present invention is applied tothe edge-emitter type laser diode in which the resonator extends in thedirection parallel to the plane of a semiconductor substrate. Thepresent invention may also be applied to a so-called vertical resonatortype (Vertical Cavity Surface Emitting Laser Diode (VCSEL)) device inwhich the resonator extends in the direction vertical to the plane of asemiconductor substrate.

FIG. 8 is a sectional view in perspective of a vertical resonance typesemiconductor laser device according to a third embodiment of thepresent invention. In the figure, reference numeral 501 designates ann-type side electrode; 502, an n-type GaAs substrate; 503, a bufferlayer; 504, an n-type semiconductor multi-layer reflection film; 505, a3-layer-structure layer consisting of a p-type cladding layer, an activelayer, and an n-type cladding layer that are layered in the order fromtop to bottom; 506, a p-type semiconductor multi-layer reflection film;507, an undoped or n-type intermediate layer; 508, an undoped or n-typecontact layer; 509, an Si film; 510, an SiO₂ diffusion protectivefilm/current block layer; 511, an Si diffusion region; 512, a Zndiffusion region; and 513, a p-type side electrode.

Current injected from the p-type side electrode 513 is blocked by abold-line portion 514. Therefore, no current leaks through a defectlayer portion 516 caused by the Si diffusion. The semiconductor laserdevice thus constructed can effectively squeeze current, to thereby emita laser beam through a window 515.

In the present invention, an AlGaInP buried laser, manufactured by themixed-crystal forming technique based on the Si impurity diffusion, can.block the leak current that would flow through dislocations and defectscaused in an upper region of the Si diffusion region in the intermediatelayer. A Si-diffusion basis, buried visible semiconductor laser deviceof the present invention, which is simpler in manufacturing process thanthe ridge-stripe type visible semiconductor laser device, can realizelow threshold current value, high efficiency, and high temperaturecharacteristic. Thus, the present invention succeeds in providing avisible semiconductor laser device of low cost and less powerdissipation.

In the above embodiments the semiconductor laser device is AlGaInPburied laser, but it is possible to apply to an AlGaAs buried laser,either. In this case, the layer structure is the same as shown in FIG.1A, and the cladding layers 105, 107 consisted of AlGaInP in the firstembodiment is substituted to AlGaAs layer, and the intermediate layer109 consisted of AlGaInP or the buffer layer 104 and the active layer106 consisted of GaInP in the first embodiment are changed to GaAs orAlGaAs layers. The conductivity type of each layer are the same as thefirst embodiment. The composition ratio of each atoms of AlGaAs layerare selected as the ratio of the conventional AlGaAs buried lasers.

What is claimed is:
 1. A semiconductor laser device, comprising:a) asubstrate; b) a first cladding layer made of semiconductor material of afirst conductivity type and layered on said substrate; c) a quantum wellactive layer made of semiconductor material and layered on said firstcladding layer; d) a second cladding layer layered on said quantum wellactive layer, said second cladding layer being made of semiconductormaterial of a second conductivity type that is opposite to said firstconductivity type; e) an intermediate layer made of semiconductormaterial of said first conductivity type or nonconductive typesemiconductor, said intermediate layer being layered on said secondcladding layer; f) a contact layer made of semiconductor material ofsaid first conductivity type of nonconductive type semiconductor, saidcontact layer being layered on said intermediate layer; g) amixed-crystal region of said first conductivity type in saidintermediate layer, said second cladding layer, and said quantum wellactive layer, said mixed-crystal region not extending under said contactlayer; and h) a low resistance region of said second conductivity typeextended over a range from said contact layer, through said intermediatelayer, to said second cladding layer, said low resistance region beingisolated from said mixed-crystal region by said intermediate layer. 2.The semiconductor laser device of claim 1, further comprising:animpurity layer made of a first impurity formed on said intermediatelayer deviated sideways from said contact layer, wherein saidmixed-crystal region is formed by diffusing said first impurity to saidintermediate layer and said quantum well active layer, from saidimpurity layer.
 3. The semiconductor laser device of claim 1, whereinsaid low resistance region is formed by diffusing a second impurity intothe structure from above said contact layer.
 4. The semiconductor laserdevice of claim 3, further comprising:a diffusion/protective layer withan opening layered on said contact layer, said second impurity beingdiffused through said opening.
 5. The semiconductor laser device ofclaim 1, wherein a band gap of said intermediate layer is larger thanthat of said quantum well active layer but smaller than that of saidsecond cladding layer.
 6. The semiconductor laser device of claim 1,wherein said intermediate layer is a superlattice.
 7. The semiconductorlaser device of claim 1, wherein said first cladding layer, said quantumwell active layer, said second cladding layer, and said intermediatelayer comprise AlGaInP mixed-crystal series.
 8. The semiconductor laserdevice of claim 1, wherein said mixed-crystal region surrounds a regionunder said contact layer, thereby forming a Vertical Cavity SurfaceEmitting Laser Diode for emitting a laser beam in a directionperpendicular to said substrate surface.
 9. The semiconductor laserdevice of claim 1, wherein the low resistance region is formed bydiffusing a second impurity.
 10. The semiconductor laser device of claim9, further comprising:a diffusion/protective layer on the semiconductorcontact layer having an opening, the second impurity being diffusedthrough the opening.
 11. The semiconductor laser device of claim 1,wherein a band gap of the semiconductor intermediate layer is largerthan semiconductor quantum well active layer but smaller than the secondsemiconductor cladding layer.
 12. The semiconductor laser device ofclaim 1, wherein the semiconductor intermediate layer is a superlattice.13. The semiconductor laser device of claim 1, wherein the firstsemiconductor cladding layer, semiconductor quantum well active layer,the second semiconductor cladding layer, and the semiconductorintermediate layer comprise AlGaInP mixed-crystal series.
 14. Thesemiconductor laser device of claim 1, wherein the mixed-crystal regionsurrounds a region under the semiconductor contact layer, therebyforming a Vertical Cavity Surface Emitting Laser Diode for emitting alaser beam.
 15. A semiconductor laser device, comprising:a firstsemiconductor cladding layer of a first conductivity type; asemiconductor quantum well active layer on the first semiconductorcladding layer; a semiconductor second cladding layer of a secondconductivity type on the quantum well active layer; a semiconductorintermediate layer on the second semiconductor cladding layer; asemiconductor contact layer on the semiconductor intermediate layer; amixed-crystal region of the first conductivity type in the semiconductorintermediate layer, the second semiconductor cladding layer, and thesemiconductor quantum well active layer, the mixed-crystal region notextending under the semiconductor contact layer; and a low resistanceregion of the second conductivity type extended from the semiconductorcontact layer, through the semiconductor intermediate layer, to thesecond semiconductor cladding layer, the low resistance region beingisolated from the mixed-crystal region by the semiconductor intermediatelayer.
 16. The semiconductor laser device of claim 14, furthercomprising:an impurity layer comprising a first impurity on thesemiconductor intermediate layer deviated sideways from thesemiconductor contact layer, wherein the mixed-crystal region is formedby diffusing the first impurity to the intermediate layer and thequantum well active layer, from the impurity layer.